page_title: Quadratojugal bone
section_title: Origin
hierarchical_section_title: Quadratojugal bone / Evolution / Origin
caption_attribution_description: English: An edited version of the source file with Mimipiscis cropped out.
context_page_description: The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians.
context_section_description: The quadratojugal likely originated within the clade Sarcopterygii, which includes tetrapods and lobe-finned fish. Although a tiny bone similar in position to the quadratojugal has been observed in the placoderm Entelognathus and some early actinopterygiians (Mimipiscis, Cheirolepis), it is unclear whether this bone was homologous to the quadratojugal. A quadratojugal is absent in actinians (coelacanths) and onychodonts, but it was clearly present in Porolepiformes, distant relatives of modern dipnoans (lungfish). Many paleontologists argue that the quadratojugal was formed by a division of the preoperculum, although a few believe that it was present before the preoperculum formed. All tetrapodomorph fish had a quadratojugal, retained by their tetrapod descendants. Elpistostegalians such as Panderichthys, Tiktaalik, and other very tetrapod-like fish were the first vertebrates to have contact between the quadratojugal and jugal. Before the elpistostegalians, the jugal was small and isolated from the quadratojugal by the squamosal and maxilla.
  Amphibians (in the broad sense) typically had long, roughly rectangular quadratojugals that contacted the maxilla, jugal, squamosal, and quadrate. In several lineages, most of them traditionally considered "Reptiliomorpha", the jugal expands downwards to reduce the amount of contact between the quadratojugal and maxilla. This is exemplified in reptiles, which have completely lost the contact. Most urodelans (salamanders) lack quadratojugals, except the Miocene genus Chelotriton. A quadratojugal is also missing in the caecilian-like Triassic stereospondyl Chinlestegophis as well as the lysorophians, a group of long-bodied Paleozoic microsaurs. Many other microsaurs had heavily reduced quadratojugals.
caption: The skull of the tetrapodomorph fish Eusthenopteron, with the quadratojugal bone labelled in pink.
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